1. Field of the Invention
The present invention relates to novel photohardenable compositions and to photosensitive materials employing them. More particularly, it relates to free radical addition polymerizable compositions involving the use of borate salts in combination with aromatic ketone photoinitiators to produce a system capable of efficient initiation of free-radical polymerization. These compounds are particularly effective when used in combination with an autoxidizer.
2. Description of the Prior Art
Dye-sensitized photopolymerizable compositions are known in the art. A survey of these systems is provided by Eaton, D. E., "Dye Sensitized Photopolymerization," Advances in Photochemistry, Vol. 13, pp. 427-87 (1985). In their simplest form these compositions include a photopolymerizable vinyl compound, a photoreducible or photooxidizable dye and an activator which functions as a reducing agent or an oxidizing agent for the dye. The dyes are excited by light to a triplet state which reacts with the reducing agent or oxidizing agent to yield radicals which can initiate polymerization. There are a number of examples of dye-sensitized photopolymerizable compositions in the patent literature. See Oster, U.S. Pat. Nos. 2,850,445 and 2,875,047 (Rose Bengal with hydrazine or thiourea); 2,547,633 (quinoidal family dyes with triorganophosphines, triorganoarsenines or sulfinic acid derivatives); 3,615,452 (phenazine or oxazine dyes and diazonium salts); 3,627,656 (phenothiazine dyes and sulfinic compounds); 3,495,987 (cyanine dyes and bromine donors) and 3,488,269 (thionine dyes and methylenes or methines).
Pursuant to the usual methods known in the art, additives are often included in the photohardenable compositions to more efficiently produce free-radicals from the photoexcited photoinitiators. Among the most popular additives are derivatives of dimethylaniline and other tertiary amines. These tertiary amine additives generally promote more efficient free-radical generation through an autoxidation cycle as described in allowed U.S. Application Ser. No. 300,168 filed Jan. 19, 1989.
U.S. Pat. No. 4,772,541 to The Mead Corporation discloses free radical addition polymerizable compositions employing a cationic dye-borate anion complex as a novel photoinitiator. The mechanism whereby this complex absorbs energy and generates free radicals is not entirely clear, however, it is believed that upon exposure to actinic radiation, the dye in the complex is excited to a singlet state in which it accepts an electron from the borate anion. The borane radical breaks down and yields a free radical. In solution in the polymerizable compound, ionic pairing of the borate anion and the cationic dye is believed to promote efficient electron transfer.
In U.S. Application Ser. No. 426,819 filed Oct. 25, 1989, the use of borate salts in combination with dye borate initiators is disclosed.